1. Field of the Invention
This invention relates to a misfire detection apparatus of an internal combustion engine for detecting the amount of ions produced by combustion in an internal combustion engine, thereby detecting occurrence of a misfire in the internal combustion engine and in particular to a misfire detection apparatus with a failure diagnosis function comprising a diagnosis function for determining whether or not the condition is a misfire detection failure caused by ion current if it is determined that the condition is a misfire based on ion current.
2. Background Art
Hitherto, for example, JP-A-10-252635 has disclosed a misfire detection apparatus using ion current detection means and comprising failure determination means based on rotation fluctuation of an internal combustion engine.
FIG. 6 is a drawing to show an ion current detection circuit in a related art example, FIG. 7 is a control block diagram in the related art example, and FIG. 8 is a flow chart of failure diagnosis in the related art example.
In FIG. 6, a circuit consisting of a Zener diode 302, a charge capacitor 303, a diode 304, and a detection resistor 305 is inserted between a secondary low-voltage terminal of an ignition coil 301 and GND. When an ignition signal is applied to an igniter 112, an electric current starts to flow into a primary coil of the ignition coil 301. If the ignition signal is reset to zero in a predetermined energization time, the current flowing into the primary ignition coil becomes zero. Energy stored in the ignition coil 301 causes the secondary coil to produce a high voltage, starting combustion. The charge capacitor 303 stores a current at the sparking time and is charged to the Zener voltage of the Zener diode 302. When spark disappears, the charge current becomes zero, but the voltage charged in the charge capacitor 303 is applied to a spark plug 107 and a discharge current into the ignition coil 301 and the periphery of the spark plug 107 flows.
Then, an ion current proportional to the ion amount in a combustion chamber flows. The ion current is integrated, whereby the combustion state can be grasped. That is, when a misfire occurs, no ions occur because of no combustion and a waveform corresponding to an ion current does not occur in output of an ion current detection circuit, but the ion current waveform occurs at the combustion time. Therefore, misfire determination means 206 determines whether or not the ion current waveform exists, whereby whether the condition is normal combustion or a misfire can be determined. That is, with the discharge start timing as a start trigger, misfire determination means 206 inputs an output signal of an ion current detection circuit 210 in a predetermined time, and compares the output signal with a threshold value, and if the output signal is equal to or less that the threshold value, determines that the condition is a misfire.
The ion current detection circuit in FIG. 6 is described as sensing means 210 in FIG. 7. A/D conversion means 205 converts analog output of the ion current detection circuit into digital form and outputs the digital signal to the misfire determination means 206. The misfire determination means 206 determines whether or not a misfire occurs in each cylinder of the engine based on the ion current value from the sensing circuit means 210, and outputs an output signal of the determination result to monitor processing means 204 and sensing circuit failure determination means 207. The monitor processing means 204 always performs monitor diagnosis processing of monitoring the relationship between the engine operation state and the control amount based on output signals from various sensors and the output signal from the misfire determination means 206 and detecting an anomaly of the various sensors or the control contents; in the monitor result, output control of fuel cutting, etc., is affected by output control means 203. The sensing circuit failure determination means 207 makes a sensing circuit failure diagnosis. In the failure diagnosis, whether or not the sensing circuit is abnormal is determined from the relationship between the combustion state determination result of the misfire determination means 206 and any other operation state based on a predetermined operation state signal and if it is determined that the sensing circuit is abnormal, a warning lamp 123 is turned on based on comparison with an output signal from a lighting condition 208, prompting the operator to make repair.
In the flowchart of FIG. 8, first at step 601, the current misfire determination is input and whether the condition is normal or a misfire is determined. If the condition is normal combustion, control proceeds to steps 610 and later, but the contents differ from the contents of the invention and therefore will not be discussed here. If it is determined that the condition is a misfire, control goes to step 602 and the time interval of a reference sensor 105 is measured and control goes to step 603. At step 603, the time interval measurement conditions of the reference sensor 105 are checked. That is, whether or not the engine speed is in a predetermined range, whether or not the vehicle speed is constant, and whether or not engine load fluctuation is in a predetermined range are determined. At step 604, if predetermined conditions are satisfied, the reference sensor interval of the cylinder wherein misfire is detected is compared with that of any other cylinder. If the interval difference is less than a predetermined value, control goes to step 605. At step 605, it is determined that the engine itself does not misfire and that the ion current detection circuit or a harness fails. If the interval difference is equal to or greater than the predetermined value, control goes to step 606 and it is determined that the engine misfires.
In the apparatus in the related art, if it is determined that the condition is a misfire based on an ion current, a failure diagnosis is made based on rotation fluctuation; to improve the failure diagnosis reliability, namely, the reliability of misfire determination based on rotation fluctuation, it is necessary to calibrate operational expressions and operational coefficients and particularly in a multi-cylinder engine, there is a problem of increasing the number of calibration steps.